For controlling nuclear reactors, in particular pressurized water nuclear reactors, clusters of materials highly absorbent to neutrons are used, these being moved vertically in the core of the reactor between the fuel elements, so as to adjust the power supplied by the reactor according to the power program required.
On the other hand, these clusters of absorbent material also serve for producing the emergency shut-off of the reactor, when all the units are caused to fall into maximum insertion position of the core of the reactor.
To achieve the movement or the falling back of these clusters of absorbent material extended upwards by an operating rod of great length, displacement mechanisms cooperating with the operating rod are used, and these are arranged inside sealed vessels communicating with the inside of the tank of the reactor within which the core is located.
These sealed vessels must permit a displacement of the operating rod corresponding to a movement of the absorbent unit between its positions of maximum and minimum insertion. The amplitude of these movements corresponds substantially to the height of the fuel assemblies, i.e., in the case of currently constructed pressurized water nuclear reactors, about 4.20 m.
The displacement mechanisms, for example pawls, are driven by a driving device which is generally arranged at the lower part of the sealed vessels which extend the tank of the reactor upwards from the cover of this tank. Consequently, the height of the sealed vessels above the pawl mechanism, cooperating with the operating rod, including notches also distributed over the length of the rod, must be at least equal to the height of the fuel assemblies.
The sealed vessels of very great height must be held at their upper part by means of an anti-earthquake device constituted by a plate itself held in position by means of tie-rods arranged on the walls of the pool of the reactor. It is extremely important, in fact, to limit stresses and distortions in the mechanisms in case of earthquakes to permit the emergency shut-down of the reactor by falling back of the clusters of absorbent material in the case where the reactor undergoes seismic shocks.
This plate holding the upper part of the sealed vessels also plays the role of anti-missile plate since it is designed to stop the sealed vessels in the case when the latter would be ejected, so as to prevent any deterioration of the adjacent equipment.
It is necessary on the other hand to create, at the level of the drive devices of the mechanisms, ventilation preventing too considerable a rise in temperature under the effect of the primary fluid filling the tank of the reactor and the sealed vessels, and to cool the drive devices to enable them to operate under good conditions.
Finally, it is also necessary to heat-insulate the cover of the tank from the outer medium, in order to avoid any loss which could lower the yield of the boiler and oblige the conditioning device, for the building which encloses the reactor, to be reinforced. The control assemblies such as described above have drawbacks due to the fact that their considerable height above the cover of the tank substantially increases the height and bulk of the latter, and that the presence of anti-earthquake devices fixed to the walls of the pool of the reactor complicates operations of opening and closing the cover of the tank.
The presence of the anti-earthquake plate above the motors and at the upper part of the sealed vessels is moreover troublesome when it becomes necessary to change a faulty motor or a part of the mechanism situated in the sealed vessel, which operation must be carried out in the presence of ionizing radiation.
On the other hand, it is necessary to use a complex and bulky heat extraction device at the level of the drive devices at the lower part of the vessels.
Another drawback is that only the cover of the tank is heat-insulated, so that there is produced a considerable heat loss at the level of the sealed vessels.